1. Field of the Invention
The present invention relates to an electrical double-layer capacitor, more particularly to an electrical double-layer capacitor using a nonaqueous electrolyte solution.
2. Description of the Related Art
An electrical double-layer capacitor comprises a structure in which a separator and an electrode element consisting of a pair of polarizable electrodes disposed opposite to each other with the separator interposed therebetween are sealed in a case, and the electrode element is impregnated with an electrolyte solution.
The polarizable electrode is formed, for example, by press molding activated carbon particles or by coating a mixture of the activated carbon particles with a suitable binder on a current collector metal. Further, an activated carbon fiber on which alumina is plasma sprayed can also be used for the polarizable electrode.
An aqueous electrolyte solution such as sulfuric acid or an aqueous potassium hydroxide solution and a nonaqueous electrolyte solution in which a quaternary ammonium salt is dissolved in an organic solvent such as propylene carbonate are known as the electrolyte solution. As the nonaqueous electrolyte solution can increase drive voltage when used for an electrical double-layer capacitor and can increase energy density compared with the case where an aqueous electrolyte solution is used, it is suitable for the applications requiring high output such as the electrical double-layer capacitor to be mounted on an electric vehicle.
A small amount of impurities contained in the nonaqueous electrolyte solution are known to reduce the withstand voltage of an electrical double-layer capacitor and degrade the capacity of the electrical double-layer capacitor. The impurities include glycols such as propylene glycol, ethylene glycol and diethylene glycol that are contained in an organic solvent such as propylene carbonate described above; a tertiary amine that is an unreacted raw material contained in the quaternary ammonium salt; and a tertiary ammonium salt as a reaction product of the tertiary amine.
Japanese Patent Laid-Open No. 2000-331887 proposes to suppress the reduction of the withstand voltage and degradation of the capacity of an electrical double-layer capacitor by reducing the content of glycols in the nonaqueous electrolyte solution to 100 ppm or less. In addition, Japanese Patent Laid-Open No. 2000-311839 proposes to suppress the reduction of the withstand voltage and degradation of the capacity of an electrical double-layer capacitor by reducing the content of a tertiary amine and a tertiary ammonium salt in the nonaqueous electrolyte solution to 2 mmol/kg or less.
However, even when using the nonaqueous electrolyte solution in which the contents of the glycols or a tertiary amine and tertiary ammonium salt are within the range described above, the electrical double-layer capacitor has the disadvantage of a large increase in resistance when used under continuous application of high voltage. Moreover, the electrical double-layer capacitor has the disadvantages of having low self-discharge properties and an insufficient energy residual ratio when it is left standing for a long period of time.